Learning about language safety from Zcoin

A recent bug in Zcoin allowed an attacker to mint ~550,000 new coins, the theoretical equivalent to USD$750,000 (and about 25% of the entire Zcoin supply). As is common with a C++ codebase, it turned out to be an easy-to-make and hard-to-spot typing problem that the permissive compiler had no qualms about letting happen (namely, a mistake of one extra character and use of a == equality operator instead of an = assignment operator).

One user on the HN comment thread asked the question that as an industry we should all be asking ourselves: “Given the well known pitfalls, is it irresponsible to start projects in C/C++?”

As is pretty normal when C++ is criticized, its proponents started coming out of the woodwork to pronounce all the things that the author had done wrong. He should have written more modular code! He should have been using -Wall! No, he should have been using -Wall -Wextra -Werrors! He should have been running Valgrind in CI!

They’re missing the point entirely. If it’s possible to introduce bugs of this severity just because you haven’t divined the use of the right tool or perscribed to a particular development doctrine, then the language is not safe. No language’s users will ever be perfectly educated, and as a result this kind of problem is bound to repeat itself until the end of time wherever it’s possible for it to happen.

Consensus on values around language design is hard to come by, but the features that newer languages all agree on seems to suggest some convergence. Compilers with strict checks are good. Types are good. Compile-time and runtime performance is good. Concurrency primitives are good. Memory safety is good. We should be aiming to write programs that are more likely to be bug-free by moving towards languages that provide these basic utilities.

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